WaterRF Project: Task 1.0

Laboratory Batch Experiments

Laboratory Batch Experiments Laboratory batch tests were conducted using sediments from two field sites to evaluate impacts of CO₂ interaction with sediments and potential release of different elements. The field sites include the Helena site in the Texas Gulf Coast and the Cranfield site in Mississippi. Sediment samples taken from the field's target aquifers were pulverized and sieved. We choose pulverized rock sediments with the same particle sizes for batch reactions. Surface area of sediment samples were calculated on the basis of particle sizes. Prior to conducting the batch experiments, we characterized the solid phase more accurately using x-ray diffraction (XRD) and scanning electron microscopy (SEM). CO₂ pressure was maintained at a constant level throughout each test. During the experiment, water samples were collected and analyzed to monitor chemical changes. At the end of the experiment, the solid phase will be examined to determine whether mineral dissolution is evident using SEM.

The inlet and outlet of the system shown in Figure 5 were controlled by a data logger which was set to purge gas for 1 minute every 20 minutes. The flasks were purged with high purity Ar gas for 5 days and then 20 mL of water was taken for chemical analysis (time=0). The Ar tank was replaced with a CO₂ tank and CO₂ gas was bubbled into the system with the same frequency as the Argon gas bubbling. It is worth noting that nine flasks were used with the same sediments and groundwater in our experiment, different than those reported in (Lu et al., 2010) and (Little and Jackson, 2010) because with nine flasks, water samples could be taken from one or two of the 9 flasks to minimize changes in water volume in a flask and avoid taking too much water from one flask which may affect water-rock-CO₂ interactions in the flask. After CO₂ was bubbled into the system, about 5 mL of water was taken from each of the nine flasks at different times. Values of pH were measured immediately after sampling using an Orion 3 pH-meter. The electrode was calibrated daily with buffer solutions at pH of 4, 7 and 10. Alkalinity was measured by titration of a sample aliquot with 0.16 or 1.6 N H₂SO₄.

XRD analyses of the sediments obtained from the Helena aquifer indicate that the aquifer sediments consist mainly of quartz (64.3%), microcline (24.8% in weight), illite 5.1%, and albite 5.9%. Trace amount of calcite were also found contained in the sediments (~0.04%). The BET surface areas of the Helena aquifer sediment is 14.42 m2/g.

Values of pH significantly decrease after CO₂ was bubbled into the water-rock system and then gradually increased slowly with time (Figure 6).

Measured groundwater alkalinity increased from 100 mg HCO₃/L to 450 mg/L and eventually reached steady state (Figure 7).

Concentrations of Mg and Ca increase with time within several days and then reach steady state (Figure 8).

This increasing trend is attributed to dissolution of carbonates contained in the sediments after CO₂ gas was bubbled into the system. Na and K concentrations show some variations after CO₂ was bubbled into the system (Figure 9).

It is interesting to note that Si concentrations show an overall increase in the first several days and then become stable. Increase in Si concentrations suggests silicate mineral dissolution. However, Na and K concentrations didn't show a corresponding increasing trend (Figure 10).

This may be due to adsorption/desorption or cation exchange reactions. Concentrations of As and V decrease after CO₂ was bubbled into the system and behave similarly (Figure 11).

This decreasing trend is attributed to adsorption processes. Concentrations of U and Mn show an overall increasing trend, which is similar to that of Ca and Mg (Figure 12).

Such a trend is likely due to co-dissolution of carbonates. Iron concentration doesn't show an obvious trend after CO₂ was bubbled into the system (Figure 13a). Concentrations of Pb, which was a major focus of research by Apps et al., 2009; 2010 do not vary significantly (Figure 13b).

 

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